In an automobile, vehicle wheels are mounted to a vehicle body via a suspension system (a suspension). One of members that constitute the suspension system is a lower arm. One end part of the lower arm is attached to the vehicle body via a frame (more specifically, a suspension member) of the suspension system. A vehicle wheel (more specifically, a wheel) is attached to the other end part of the lower arm. From the viewpoint of reduction in weight, it is desirable to adopt a press-formed product to the lower ann.
FIG. 1 and FIG. 2 are perspective views schematically showing shape examples of the press-formed products that are usable as lower arms. Among these Figures, FIG. 1 shows a press-formed product of a first example, and FIG. 2 shows a press-formed product of a second example. Press-formed products 10 shown in FIG. 1 and FIG. 2 each includes a main body part 11 and a protruded part 12. The main body part 11 extends in an L-shape or a bow-shape along a lengthwise direction in plan view. Of both ends of the main body part 11, one end part (hereinafter, also referred to as “a first end part”) 11a is an end part that is attached to an automobile vehicle body. Hereinafter, the first end part 11a will also be referred to as “the vehicle body attaching end part”. The other end part (hereinafter, also referred to as “a second end part”) 11b is an end part that is attached to an automobile vehicle wheel. Hereinafter, the second end part 11b will also be referred to as “the vehicle wheel attaching end part”. In each of FIG. 1 and FIG. 2, a side to be connected to the vehicle wheel will be designated by reference sign “W”, whereas a side to be connected to a vehicle body will be designated by reference sign “B”.
The protruded part 12 protrudes outward of bending from a bent part of the main body part 11. FIG. 1 and FIG. 2 each show an embodiment in which the protruded part 12 is provided substantially in a middle in the lengthwise direction of the main body part 11. As well as the first end part 11a of the main body part 11, the protruded part 12 is a part that is attached to an automobile vehicle body.
Sectional shapes of the main body part 11 and the protruded part 12 are both groove-shaped. That is, the main body part 11 and the protruded part 12 each include top plate parts 13a and 13b, and vertical wall parts 14a, 14b and 14c. The vertical wall parts 14a to 14c each extend from opposite sides of the top plate parts 13a and 13b. On a surface of the top plate part 13a of the main body part 11, a groove part 15 along the lengthwise direction of the main body part 11 is provided.
In a case of the press-formed product 10 of the first example shown in FIG. 1 , the groove part 15 is provided in a domain on the first end part (the vehicle body attaching end part) 11a side from a root of the protruded part 12 in the surface of the top plate part 13a. That is, the groove part 15 is provided to a vicinity of the first end part 11a from a vicinity of the root of the protruded part 12. The groove part 15 becomes deeper toward the first end part 11a. Consequently, a bottom surface of the groove part 15 is an inclined surface. Note that the depth of the groove part is made constant without inclining the bottom surface of the groove part in some cases.
In the case of the press-formed product 10 of the second example shown in FIG. 2, the groove part 15 is provided in a domain from the first end part (the vehicle body attaching end part) 11a to the second end part (the vehicle wheel attaching end part) 11b so as to pass through the root of the protruded part 12, in the surface of the top plate part 13a of the main body part 11.
Among the vertical wall parts 14a to 14c, the vertical wall part 14a that extends from the first end part (the vehicle body attaching end part) 11a of the main body part 11 to the protruded part 12 has a bay-shaped corner part 16 that connects the main body part 11 and the protruded part 12. Hereinafter, the vertical wall part 14a will also be referred to as “the first vertical wall part”. A ridge part 17 is formed by the first vertical wall part 14a and the top plate parts 13a, 13b (refer to the thick line portions in FIG. 1 and FIG. 2). In the ridge part 17, a portion overlaid on the corner part 16 is in a circular-arc shape. In the ridge part 17, an angle that is formed by a portion which is on the main body part 11 side from the corner part 16 and a portion which is on the protruded part 12 side from the corner part 16 is an acute angle.
Hereinafter, the vertical wall part 14b that extends from the first end part (the vehicle body attaching end part) 11a of the main body part 11 to the second end part (the vehicle wheel attaching end part) 11b of the main body part 11 will also be referred to as “the second vertical wall part”. The vertical wall part 14c that extends from the second end part (the vehicle wheel attaching end part) 11b of the main body part 11 to the protruded part 12 will also be referred to as “the third vertical wall part”.
The press-formed products 10 in the shapes as above are formed by applying press-working to blank metal plates. In accordance with necessity, restriking is applied to a region that cannot be formed by press-working, such as a very small R part, and thereby shape finish or shape fixation are performed. Further, for the purpose of finishing to a final shape, trimming, boring, welding and the like are applied in accordance with necessity.
When the lower arm as a press-formed product is produced, the type of forming that is applied to a blank metal plate is mainly stretch flanging. The prior arts concerning stretch flanging include the following.
Japanese Patent Application Publication No. 2011-230189 (Patent Literature 1) discloses a technique of performing press-working in a warm condition by heating a steel plate. In the technique of Patent Literature 1, a high-strength steel plate of a precipitation strengthening type is used as the steel plate, and performs press-working of the steel plate which is heated to a predetermined temperature T at an average forming speed of 200 mm/s or more. At this time, the heating temperature T of the steel plate and a softening temperature TL of the steel plate satisfy a relation of TL−100≤T≤TL. Patent Literature 1 indicates that thereby, even when a high-strength steel plate which is difficult to subject to press-working is used as a starting material, a press-formed product can be produced without reducing production efficiency.
Japanese Patent Application Publication No. 2009-160655 (Patent Literature 2) discloses a technique for producing a press-formed product with a flange (a vertical wall part). In the technique of Patent Literature 2, as the blank metal plate, a blank is used, which is composed of a blank part for a substrate corresponding to a flat-shaped substrate, and a blank part for a concave-shaped flange that is bending-formed into a concave-shaped outer circumferential edge of the blank part for a substrate. The concave-shaped blank part is constituted of a blank part for a convex-shaped flange where stretch flanging deformation occurs, and a blank part for an adjacent flange that is adjacent to the blank part for a convex-shaped flange. An outer circumferential edge of the blank part for a convex-shaped flange is formed in a predetermined range. Patent Literature 2 indicates that thereby, tensile stress concentration that occurs to the convex-shaped flange part of the press-formed product can be dispersed to both end portions, and a stretch flange crack can be suppressed.
Japanese Patent Application Publication No. 06-87039 (Patent Literature 3) discloses a technique of burring that forms a cylindrical part at a circumferential edge of a hole. In the technique of Patent Literature 3, drawing is performed by setting a draw radius to be large in a first step. In a subsequent second step, restriking is applied to the draw part formed in the first step, and an entire bottom part of the draw part is further pierced at a last stage of the work. Thereby, a vertical wall part of a remaining draw part by piercing of the bottom part directly becomes a cylindrical burring part, and stretch flanging is not needed. Patent Literature 3 indicates that consequently, even if the draw radius is set to be large, and a drawing height is enlarged to a work limit, a problem in forming such as a crack does not occur.